This is a competing renewal of a highly succesful project concerning the role of the Hh pathway in medulloblastoma. Previously, we obtained dramatic results using a small molecule antagonist of the Hh pathway (HhAntag) in a spontaneous mouse model of medulloblastoma. However, recently, we also found that HhAntag treatment of young mice causes severe defects in bone development. We will now investigate the mechanism of action of HhAntag in tumor cells, as well as in developing bone, to optimize strategies for delivering this potentially revolutionary treatment for a subset of human medulloblastoma. We will also establish new pediatric brain tumor models to investigate the mechanism of action of HhAntag in human medulloblastoma as well as in a broader spectrum of pediatric brain tumors. In Aim 1, we will address the mechanism responsible for the anti-tumor effect of HhAntag as well as its effect on bone development. We will investigate alternative delivery methods and different structural classes of inhibitors in an effort to minimize the deletrious effects on developing bone. We will also screen for other inhibitors or targets that may be more selective for brain tumors. In Aim 2, we propose to investigate the role of the microenvironment in maintaining Hh pathway activity in vitro. We will utilize stem cell culture techniques, coculture with stromal support cells, and transplantation in an attempt to maintain pathway activity in mouse medulloblastoma cells. Once established, we will use these conditions to propagate human medulloblastoma carrying PTCH1 mutations so that we can test their response to HhAntag. In Aim 3, we will investigate the role of the Hh pathway in pediatric brain tumors lacking known Hh pathway mutations. Tumors will be collected and characterized by genomic profiling, and used to establish new culture and transplantation models. We will investigate the effect of HhAntag on tumors and stromal support cells in a range of tumors, including Atypical Teratoid/Rhabdoid Tumors and Choroid Plexus Carcinoma.